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Three-Dimensional Model of Tien-Shan Litosphere

#KR-837


Three-Dimensional Velocity Model of Tien-Shan Litosphere in Relation to Geodynamics and Seismic Hazard

Tech Area / Field

  • OBS-NAT/Natural Resources and Earth Sciences/Other Basic Sciences

Status
8 Project completed

Registration date
05.12.2001

Completion date
07.08.2006

Senior Project Manager
Isakov S V

Leading Institute
Institute of Seismology, Kyrgyzstan, Bishkek

Collaborators

  • Rensselaer Polytechnic Institute, USA, NY, Troy\nIndiana University / Dept. of Geological Sciences, USA, IN, Bloomington\nGeoForschungsZentrum Potsdam, Germany, Potsdam\nUniversity of Colorado / Department of Geological Sciences and Cooperative Instiute for research In Environmental Sciences (CIRES), USA, CO, Boulder

Project summary

The aim of the Project is to refine 3D velocity model of Tien-Shan crust and upper mantle (litosphere) and provide its geological and geophysical interpretation; use of this model for perceiving a nature of regional dynamic processes as well as their particular manifestation – seismic activity for improvement seismic hazard evaluation method at Kyrgyzstan area and earthquakes accurate location.

A principal task of the Project is computaion of new 3D velocity model of Tien-Shan litosphere for depths of 0-350 km. First 3D velocity model of litosphere, computed 10 years ago (Roecker, Sabitova,Vinnik et al., 1993), played specific role in structural investigations, but calculated on limited data set it remained underinformative. At present a necessity has appeared to refine model using extended data set and significantly improved KNET (Northern Tien-Shan) observation system as well as temporal broad-band stations array in mountain regions of Middle and Southern Tien-Shan difficult to access. Later array was deploid by S.Roecker within the National Science Foundation project «Geodynamics of intracontinental mountain building in Tien-Shan, Central Asia» and operated during 1999-2000. New 3D velocity model will comprehend region enclosed in latitude 38°- 44°N and longitude 68°-80° E covering underresolved earlier in 1993 areas such as seismically active Kok-Shaal and Alay.

Study of foregoing region 3D velocity structure has already been attempted within ISTC Project #KR-155 «Velocity structure and media's elastic properties in source zone of Tien-Shan strong earthquakes» (Sabitova, Adamova, 2001) but only up to 85 km depth, including Tien-Shan crust and uppermost mantle. Project completed in May, 2001. However to perceive geodynamics processes ongoing in Tien-Shan, where upper mantle plays major role, we need to look deep inside and try to reconstruct Tien-Shan litosphere interior up to 300-350 km depths.

Analysis of Tien-Shan litosphere 3D velocity structure together with its geological and geophysical interpretation, use of other techniques such as receiver function, S-wave coda attenuation as well as data of tectonics, P-T conditions analysis, earthquake focal mechanisms we anticipate will contribute a lot into solution of such fundamental problem as understanding of dynamic processes, causing mountain building and seismic activity.

Another important aspect of problem deals with applied character of researches suggested. Seismic hazard assessment and mitigation of devastating earthquake aftermath still remain a task of urgency for Kyrgyzstan with its high seismic activity. It is a task of special concern in view of UN decision to declare 2002 as the Year of Mountains. Knowledge of structural features of media in which earthquakes origin would facilitate prediction of possible earthquake location and size and therefore to solve the problem of seismic hazard assessment and dizaster mitigation to a certain degree. Study we suggest will enable to find out features of velocity structure and tectonic setting which are specific for known earthquakes and develop new criteria for revealation of possible earthquake location in future and herewith to improve methodology for Kyrgyzstan area seismic hazard evaluation.

One of the Project aspects is accurate location of the earthquake and selection of reference events – earthquakes with location uncertainty of 5-10 km (GT-5 – GT5-10). In view of criteria elaborated by IASPEI Work Group on Reference Events, till now we could provide IASPEI Data Center with only 10 rather accurate earthquake hypocenters (GT-5). All those events are in Northern and south-west part of Southern Tien-Shan where majority of regional and KNET seismic stations available, providing a good azimuthal coverage. Earhquake in the other Tien-Shan areas could accurately be located only with the use of temporal US digital stations, operated in 1999-2000 in Middle and Southern Tien-Shan, which provide necessary station coverage. Hypocenters will be located in new 3D velocity structure of litosphere. This research will allow Kyrgyz seismologists to improve Central Asia Catalogue of earthquakes and based on accurate locations to make contribution into testing and further refining global velocity models.

To achieve foregoing purpose one need to solve following tasks:


1 .Mastering digital data processing techniques for analysing earthquakes, recorded by stations located within Kyrgyzstan area.
2. Computation 3D velocity model of Tien-Shan litosphere by seismic tomography techniques.
3. Acquisition of new knowledge about location of major conversion boundaries in earth crust and upper mantle by using recever functions.
4. Study of media's attenuation properties using S-wave coda attenuation.
5. Geological mapping (active faults, paleoseismodislocations etc.)
6. Analysis and interpretation of 3D velocity models of Tien-Shan litosphere, supplemented with tasks 3-5 results.
7. Analysis and interpretation of velocity crossections.
8. Development of criteria for distinguishing possible strong earthquake zones and their application to seismic hazard evaluation for some areas of Kyrgyzstan.
9. Study of focal mechanisms for Tien-Shan earthquakes.
10. Study of Tien-Shan geodynamics, getting new knowledge on dynamic processes nature.
11. Relocation of earthquakes, recorded by analog and digital networks, in 3D Tien-Shan litosphere velocity model, selection of reference events GT-5 – GT-10.
12. General analysis of results obtained.

Methodology: principal – seismic tomography, supplementary – receiver function, attenuation of S-wave coda, focal mechanisms.

Experimental data: Arrival times of Р and S-waves from regional earthquakes and explosions (epicentral distances D = 0-600 km); travel times of Р, рS and sP waves from teleseismic earthquakes (D= 30°-65°); coda envelops of S-waves from local (D about 50 km) earthquakes, data on Р-Т-conditions, focal mechanisms, geological schemes (active faults, paleoseismodislocations etc.).

Observation system: analog stations located in Kyrgyzstan and adjacent regions, american broad-band stations (KNET and temporary array, operated in 1999-2000). Altogether about 300 stations were operating in various time during 40 years.


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